The invention relates to a method for operating a four-stroke reciprocating-piston internal combustion engine of the generic type.
Reciprocating-piston internal combustion engines afford the possibility, during the compression ignition of homogeneous lean mixtures, of the formation of only small amounts of nitrogen oxides, along with high efficiency, when throttle control and a rich mixture are avoided. Compression ignition functions only with exhaust-gas retention in the case of compression ratios which are customary in engines. At a low engine load and low engine speed, and also when the engine is cold, however, even the rise in temperature due to exhaust-gas retention is not sufficient for the reliable ignition of the fresh charge.
DE-A 195 19 663 describes a method for operating an internal combustion engine with compression ignition. Here, in a first step, a homogeneous and lean air/fuel mixture generated as a result of external mixture formation is compressed to near the ignition limit. In a second step, an additional quantity of the same fuel is finely atomized and, with wall contact being avoided, is injected into the combustion space. The fuel injected late forms a mixture cloud which ignites, since, because of the higher fuel content, its ignition limit is below the compression temperature reached in the first step. These conditions apply to a higher engine load and higher engine speed and to an engine which is running hot.
The object on which the invention is based is to provide a method of the generic type which allows reliable ignition and low-consumption and low-pollutant combustion even at a low load and a low engine speed and when the engine is running cold.
By virtue of the method according to the invention, the reciprocating-piston internal combustion engine ignites the mixture of air and fuel by virtue of the rise in temperature of the mixture. The rise in temperature of the fresh mixture is brought about by mixing with retained exhaust gas from the previous cycle and due to the subsequent geometric compression of the closed-off maximum initial volume to a remaining residual volume. In the compressed volume, a temperature is established which brings the mixture to ignition. The combustion process which follows the compression ignition of the homogeneous lean mixture is a process which is self-maintained maintained due to the energy released.
The exhaust gas occurs in the combustion space as a result of the combustion of the fresh mixture. The energy released during combustion is dissipated as a result of expansion up to the maximum combustion-space volume. Subsequently, an outlet member is opened and exhaust gas is expelled as a result of the reduction in the combustion-space volume. While the expulsion operation is taking place, during the reduction in the combustion-space volume, the outlet member closes and retains the exhaust gas. The latter is compressed to the minimum combustion-space volume. The retained exhaust gas has occurred during combustion with air excess. The combustion phase is in the region of maximum geometric compression between the compression phase and the expansion phase. The number of thermodynamic phases of the four-stroke engine therefore amounts to five phases.
In order to broaden the operating range of an engine with the compression ignition of homogeneous lean mixtures, a sixth thermodynamic phase, the activation phase, is interposed. During the compression of the retained exhaust gas, an activation fuel quantity is injected into the air/exhaust-gas mixture and is distributed as homogeneously as possible, together with the remaining air fractions, in the combustion space. Thermal energy is supplied to the fuel by conduction and compression, so that a chemical reaction and/or ignition is initiated. As a result of complete combustion of the activation fuel, the thermal energy of the exhaust gas which has remained is increased in order to ensure ignition in the next cycle. If combustion is incomplete, at least the chemical activity of the retained exhaust gas quantity is increased (the formation of radicals), without the temperature being raised appreciably at the same time. In both cases, and in a situation where there is a mixture of these, it is possible to speak of activation. A greater fresh-charge mass can be ignited by means of a smaller mass of the retained exhaust gas due to the activation of the latter.
In the case of combustion during exhaust-gas compression, there is a rise in the state of pressure and temperature of the retained exhaust gas in the combustion space. This type of activation is also referred to cascadic combustion, since combustion takes place over two cycles. This has to be taken into account in a later inlet trigger time, in order to ensure a negative pressure difference of the inlet valve due to a longer expansion of the activated exhaust gas. There is no provision for an accumulation of exhaust gas or for pushing the charge back into the suction pipe, since the exhaust-gas quantity necessary for initiating a reaction loses its specific enthalpy due to flow movements and mixing movements.
At low engine speeds, that is to say with high heat losses at the wall, the aim is to achieve maximum activation of the exhaust gas. There is the risk, in this case, that, during exhaust-gas compression, complete combustion will raise the pressure level in the combustion space in such a way that the pressure difference between the surroundings and the combustion space is subsequently not sufficient to suck in a sufficient quantity of fresh gas in the time which has remained when the inlet valves are open. In order to control the pressure and temperature of the retained exhaust gas in the combustion space, the injection point of the activation fuel is varied. If injection is early, that is to say takes place even during the reduction in the combustion-space volume, the temperature of the mixture is increased due to compression, with the result that a reaction is initiated, along with subsequent combustion. If injection of the activation fuel takes place in the region of the minimum combustion-space volume, the increase in this volume delays chemical activity (the formation of radicals), without a pronounced temperature rise being capable of being established in the exhaust gas as a result of self-maintaining reactivity.
The evaporation energy of the fuel injected late extracts thermal energy from the compressed retained exhaust gas. The evaporation energy may also be used to prevent uncontrolled after-ignition of the incompletely burnt mixture mass during the compression of the retained exhaust gas. The chemical activation of the exhaust gas brings about an early initiation of the main reaction in the subsequent complete compression phase and combustion phase. The ignition point of the fresh charge during main combustion can be controlled by means of the point and quantity of activation injection.
In this way, an incomplete reaction during main combustion can be prevented by means of the control action exerted by a cooling ignition injection of activation fuel. The effects of integral influences due to the transmission of wall heat on combustion can be seen over several cycles. The influence of the transmission of wall heat within a cycle can be compensated for a number of subsequent cycles by means of a controlled action involving the variation in activation. This control for combustion stabilization requires its own control loop.
Incomplete combustion does not increase the emission of exhaust gas from the entire engine system, since the inlet control times are always selected in such a way that only fresh gas is sucked in. A possibly incomplete chemical reaction during exhaust-gas compression or expansion is implemented completely as a result of subsequent compression with a high effective compression ratio and subsequent conversion. Activation is used when cyclic combustion can no longer be maintained in the event of a further increase in the fuel fraction in the fresh mixture or when there is exhaust-gas retention. This is recognized by erratic running or a load drop when the engine is in operation.
The logics in the control of ignition injection attempt to keep the ignition injection quantity minimal with a view to a combustion configuration with optimum consumption. The control limits the maximum injection quantity by means of the signal for air deficiency of the exhaust gas (air/fuel ratio xe2x89xa61) from the lambda probe in the exhaust-gas tract.
Effective compression of the residual gas is dependent on the exhaust-gas fraction retained. It is, in most cases, at or below half the geometric compression. On account of the low fuel quantities for activation and of the still relatively high exhaust-gas fraction, during ignition combustions the peak pressures and peak temperatures in the exhaust gas are also so low that nitrogen oxides cannot occur in any appreciable quantity.